Influence of sewage discharge on dissolved oxygen concentration and fish diversity in the Girado stream and Chascomús lake.

Hypoxic aquatic environments have increased in recent decades mainly by human activities that generate pollution. The objective of this work was to study the influence of the discharge of sewage effluents from Chascomús city on the concentration of dissolved oxygen (DO) in the Girado stream and its connection with Chascomús lake and analyze the impact on fish biodiversity. Four sites were selected to measure DO and temperature weekly for one year: discharge of sewage effluent, union of the sewage effluent with the Girado stream, Girado stream and Chascomús lake. Also, ichthyological samplings were carried out in a presumed hypoxic zone and a normoxic zone of the Girado stream. The results showed that there were two hypoxic zones corresponding to the sewage effluent and the union of the effluent with the Girado stream (~3 mg l-1). Also, two normoxic zones were identified, the course of the Girado stream and the union with Chascomús lake (~ 8 mg l-1). In these areas the water temperature varied according to the seasonality of the region (from 10 to 30 °C) without differences between zones. It is important to note that these areas received a constant flow of sewage effluent from Chascomús city (~ 885,600 l per day). In the fish sampling carried out in the 4 seasons of the year, 14 typical species of the Pampas lakes were captured with species more tolerant to low DO. Pejerrey were captured in a small number in both sites but with no signs of abnormalities in the gonads.

Gonadal development in pejerrey (Odontesthes bonariensis) during spawning season in relation with sex steroids and temperature variation in Gómez lake (Pampas region, Argentina).

Gómez lake (34 ° 39 'S 61 ° 01' W) is a typical shallow lake of Pampas region placed in the upper area of the Salado river. The most abundant fish species in this lake is the pejerrey (Odontesthes bonariensis) valued due to the quality of its flesh and its attractiveness as a game fish. The aim of this study was to describe for the first time in this pejerrey wild population the gonadal stages during three consecutive spawning seasons (August to December) in relation with sexual steroids and temperature in this lake. In general, pejerrey gonadal development, the gonadosomatic index and the plasma levels of estradiol and testosterone varied in relation to air temperature. During the sampling period, pejerrey females started to ovulate in early August (winter), with a peak in October and ending in December with some of them with atretic oocytes. For males, it was possible to find spermiating animals during the whole spawning season and some arrested animals only in December. Our results confirm the relationship with pejerrey maturation and temperature and can be useful for decision making in the management of this natural resource.

Evidence of a landlocked reproducing population of the marine pejerrey Odontesthes argentinensis (Actinopterygii; Atherinopsidae).

In South America, the order Atheriniformes includes the monophyletic genus Odontesthes with 20 species that inhabit freshwater, estuarine and coastal environments. Pejerrey Odontesthes argentinensis is widely distributed in coastal and estuarine areas of the Atlantic Ocean and is known to foray into estuaries of river systems, particularly in conditions of elevated salinity. However, to our knowledge, a landlocked self-sustaining population has never been recorded. In this study, we examined the pejerrey population of Salada de Pedro Luro Lake (south-east of Buenos Aires Province, Argentina) to clarify its taxonomic identity. An integrative taxonomic analysis based on traditional meristic, landmark-based morphometrics and genetic techniques suggests that the Salada de Pedro Luro pejerrey population represents a novel case of physiological and morphological adaptation of a marine pejerrey species to a landlocked environment and emphasises the environmental plasticity of this group of fishes.

A standard weight equation to assess the body condition of pejerrey Odontesthes bonariensis.

We developed a standard weight equation Ws to aid in the analysis of pejerrey Odonthestes bonariensis body condition over time and across populations using the relative weight index Wr. Weight--length data were compiled from 73 populations of pejerrey (N=16.022) from the Argentine pampas region. We used the regression-line-percentile technique, which provides a 75th-percentile standard by length intervals of 10 mm, to develop the Ws equation. The proposed equation is log 10 Ws=-5,267+3,163 log10 Lst; Ws is weight in grams and Lst is standard length in millimeters. This equation is proposed for use with pejerrey between 120 and 520 mm of Lst. Values for Wr calculated with the Ws equation did not consistently increase or decrease as function of fish length, indicating absence of length bias. We analyze the values and distribution of Wr for pejerrey and suggest how to interpret its results. The equation of Ws that intends to calculate the index of Wr, represents a useful tool of analysis, because not only it allows to statistically compare the physical condition of the pejerrey, independently of its size, capture moment or the individual origin, but also it facilitates to relate it with other variables.

Aporte de la Estación Hidrobiológica de Chascomús al Proyecto de educación Ambiental y ecología lacustre

En este trabajo se presentan muestreos limnológicos e ictiológicos realizados por los alumnos de la Escuela Media No. 3 de Chascomús que participaron del PEAEL juntamente con profesionales de la Estación Hidrobiológica de Chascomús. Esta muestras fueron tomadas en la Laguna de Chascomús dentro del Plan de Monitoreo de Lagunas Bonaerenses para dar pautas de manejo que desarrolla la Estación. La metodología empleada consistió en medir la profundidad, la temperatura y transparencia del agua; tomar muestras de plancton y realizar capturas de peces para determinar las especies que habitan la laguna